1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to papermaker""s fabrics, namely the forming, press and dryer fabrics, also known as paper machine clothing, on which paper is manufactured on a paper machine. In addition, the present invention may be applied in other industrial settings where industrial belts are used to dewater a material.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, on a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulose fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
It should also be appreciated that the vast majority of forming, press and dryer fabrics are, or at least include as a component, a woven fabric in the form of an endless loop having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, paper machine clothing manufacturers are required to produce forming, press and dryer fabrics to the dimensions required to fit particular positions in the forming, press and dryer sections of the paper machines of their customers. Needless to say, this requirement makes it difficult to streamline the manufacturing process, as each fabric must typically be made to order.
Moreover, because the surface of a woven fabric is necessarily uneven to some degree, as knuckles formed where yarns lying in one direction of the fabric wrap around those lying in another direction lie on the surface, it is difficult to produce a paper product entirely free of sheet marking.
The prior art includes several attempts to solve these problems. For example, U.S. Pat. No. 4,495,680 to Beck shows a method and apparatus for forming a base fabric composed solely of warp yarns to be used in making a papermaker""s felt. Essentially, the warp yarns are helically wound about two parallel rolls. Subsequently, fibrous batting or other nonwoven material is applied and adhered to the helical array of warp yarns to provide a fillingless papermaker""s felt, which is to say that it has no cross-direction yarns. By eliminating cross direction yarns, the drainage characteristics of the felt are said to be improved, and pressure points caused by yarn crossovers are said to be eliminated.
U.S. Pat. No. 4,537,658 to Albert shows a papermaker""s fabric made from a plurality of elongated, linked, slotted elements. The elongated elements can be formed by extrusion or by lamination, and are linked one to the next either by an integral tongue or through the use of a pintle connecting means which extends from one elongated element to the adjacent element. The elongated elements extend in the cross-machine direction of the disclosed papermaker""s fabrics, and have flat, parallel top and bottom surfaces.
U.S. Pat. No. 4,594,756 to Beck also shows a method and apparatus for forming a base fabric composed solely of warp yarns to be used in making a papermaker""s felt. The method and apparatus shown are improved relative to those disclosed in U.S. Pat. No. 4,495,680, which was discussed above. As in the earlier patent, the endless base fabric, or substrate, produced is composed solely of machine-direction yarns and can subsequently be needled with fibrous batting to produce all or a part of a papermaker""s felt or other fabric.
U.S. Pat. No. 4,842,905 to Stech shows a tessellated papermaker""s fabric and elements for making the fabric. The elements are formed so as to have male or projection members which interlock with female or recess members. The papermaker""s fabric comprises a plurality of the tessellated elements which have been interconnected to produce a tessellation of a desired length and width. The fabrics are said to be designed to produce desired air and moisture permeabilities and drainage characteristics while providing an increased control over the paper-carrying surface thereof.
The present invention provides an alternative solution to the problems addressed by these prior-art patents.
Accordingly, the present invention is a fabric for the forming, press and dryer sections of a paper machine. The fabric may also be used as a reinforcing base for a polymeric-resin-coated paper-processing belt, such as a sheet-transfer, long nip press (LNP) or calender belt, and as part of other industrial process belts, such as corrugator belts. Moreover, the fabric may be used in other industrial settings where industrial belts are used to dewater a material. For example, the present invention may be used as a pulp-forming or pulp-pressing fabric; as a fabric used to dewater recycled paper during the deinking process, such as a dewatering fabric or belt on a double-nip-thickener (DNT) deinking machine; or as a sludge dewatering belt. The fabric is in the form of an endless loop, and has an inner surface and an outer surface.
The fabric comprises a monofilament yarn of non-circular cross section, which monofilament yarn has a length, an upper surface and a lower surface. Preferably, the upper and lower surfaces are flat and parallel to one another. Alternatively, the upper and lower surfaces may be convexly or concavely rounded in their widthwise directions. The monofilament yarn also has a first side and a second side, which are oppositely and correspondingly shaped.
The monofilament yarn is spirally wound in a plurality of turns wherein the first side of the monofilament yarn fits against the second side of an adjacent abutting spiral turn thereof. Adjacent spiral turns of the monofilament yarn are secured to one another at the abutting first and second sides to form the fabric by a variety of means.
The monofilament yarn may have cooperating first and second sides having any one of several different opposite and corresponding shapes. Specifically, the first side may have a cylindrically convex shape while the second side may have a cylindrically concave shape of the same radius of curvature, so that the first and second sides of adjacent turns of the monofilament can fit closely together when abutted against one another. Alternatively, the first and second sides may be planar, and slanted in parallel directions, so that the first and second sides of adjacent turns can fit closely together when abutted against one another.
The first and second sides of the monofilament yarn may alternatively form a tongue-in-groove joint, or may have snappingly engagable male and female mutually interlocking members, respectively. The first and second sides may also have portions of reduced thickness which overlap one another when the monofilament yarn is spirally wound to produce the fabric. Attachment of adjacent abutting turns of the monofilament yarn to one another may be made through the use of an adhesive, which may be a heat-activated, room-temperature-cured (RTC) or hot-melt adhesive, where the first and second sides are not provided with interlocking members.
The monofilament yarns may be provided with holes passing therethrough from their upper to their lower surfaces for the drainage of water from a cellulosic fibrous web being conveyed by the fabric. The monofilament yarns may also be hollow, providing them with resiliency as well as void volume for the temporary storage of water. Where this is the case, the monofilament yarn may have holes connecting the upper surface of the monofilament yarn to the void volume, and may also have holes connecting the lower surface of the monofilament yarn to the void volume. In any event, the holes may be produced by needling or by laser xe2x80x9cpunchingxe2x80x9d, or may be formed during extrusion.
One or both of the upper and lower surfaces of the monofilament yarn may be provided with grooves for the temporary storage of water. The grooves may be provided during the extrusion of the monofilament yarns; that is, they may be provided by the die used to extrude the yarns. The grooves may also be provided through the use of a slitter knife or the like while the monofilament yarn is being spirally wound to form the fabric or at any time thereafter.
The fabric of the present invention may be included as part of a multi-layered laminated fabric comprising two or more layers produced from the monofilament yarn. Alternatively, the fabric may be laminated with a woven base fabric, or with unwoven systems of longitudinal and/or transverse yarns.
The fabric, when used as a press fabric in the press section of a paper machine, may also be provided with a staple fiber batt attached to either one or both of its inner and outer surfaces by needling or by an adhesive, which may be a heat-activated, room-temperature-cured (RTC) or hot-melt adhesive, or by needle punching. Layers of non-woven fibrous material, such as those manufactured by Sharnet, can be used in addition to, or as a substitute for, some portion or all of the staple fiber batt. These can include spun bonds, melt blowns and the like.
The present invention will now be described in more complete detail with frequent reference being made to the figures identified below.